1. Field of the Invention
The present invention relates to a method of driving a liquid crystal element mounted on a display device or the like and, more particularly, to a method of driving a ferroelectric liquid crystal element.
2. Related Background Art
An electrooptical element using a ferroelectric liquid crystal (to be referred to as an FLC) has been applied to mainly a simple matrix display element because it responds to an electric field at a high speed and exhibits bistability. In recent years, however, the study of an application of the FLC element to an active matrix display element has begun. One characteristic feature of the active matrix FLC element is that a scanning time (frame period) of one frame can be determined regardless of the response speed of the FLC. In the simple matrix FLC, since the liquid crystal must respond within a selection time for one scanning line, a frame period cannot be decreased to be less than (the response speed of the liquid crystal).times.(the number of scanning lines). Therefore, as the number of scanning lines is increased, the frame period is undesirably prolonged. In contrast to this, in the active matrix FLC, only charging/discharging of pixels on one scanning line need be performed within a selection time of the scanning line, and a switching element of the pixels is turned off to hold an application voltage to the liquid crystal after the selection time. Therefore, the liquid crystal responds within this holding time. For this reason, since the frame period is independent from the response speed of the liquid crystal, the active matrix FLC can operate at a speed of 33 ms that is used in normal television sets even if the number of scanning lines is increased.
The second characteristic feature of the active matrix FLC is easiness in tone display. One tone display method of the active matrix FLC is described in EP 284,134, and the principle of the method is that pixels are reset in one stable state beforehand and a charge amount Q is applied to a pixel electrode through an active element, thereby partially causing switching to the second stable state in one pixel. When this principle is used, assuming that an area in which the switching to the second stable state is caused is a and the magnitude of spontaneous polarization of the FLC is P.sub.S, an electric charge of 2P.sub.S .multidot.a is moved upon switching, and the switching to the second stable state continues until this electric charge cancels the electric charge Q applied first. Finally, an area of EQU a=Q.sqroot.2P.sub.S
is set in the second stable state. The control of a, i.e., an area tone is realized by changing Q.
According to the experiments conducted by the present inventors, however, the above area tone method using the charge modulation has one drawback in that transition from the first to second stable state does not progress but stops until the electric charges completely cancel each other as described above. This state is shown in FIGS. 4A and 4B. FIGS. 4A and 4B plot changes over time in inter-pixel electrode voltage (FIG. 4A) and transmitted light intensity (FIG. 4B) obtained when the reset and the tone display are repeated at a period of 33 ms as in a normal television set. The voltage is abruptly attenuated immediately after the active element is turned off, but then the attenuation becomes very moderate. Similarly, although the transmitted light intensity is abruptly changed immediately after the active element is turned off, the change gradually becomes moderate. That is, although an electric field is present between the electrodes, the reversal between the two states progresses only very slowly or stops.
Because of this phenomenon, a residual DC electric field is continuously applied on the liquid crystal to lead to degradation in the liquid crystal material. Alternatively, as shown in FIG. 5, in a liquid crystal element in which an insulating layer is formed between an electrode and a liquid crystal, impurity ions in the liquid crystal are adhered on the interface of the insulating layer by a DC electric field to generate an electric field in a direction opposite to the DC electric field, thereby degrading the bistability of the FLC.
FIG. 5 is a sectional view showing a practical example of a ferroelectric liquid crystal cell using a TFT to be used in the present invention.
Referring to FIG. 5, a semiconductor film 26 (e.g., amorphous silicon doped with hydrogen atoms) is formed on a substrate 30a (e.g, glass or plastic material) via a gate electrode 34 and an insulating film 32 (e.g., a silicon nitride film doped with hydrogen atoms), and a TFT constituted by two terminals 18 and 21 in contact with the semiconductor film 26 and a pixel electrode 22 (e.g., ITO: Indium Tin Oxide) connected to the terminal 21 of the TFT are also formed on the substrate 30a.
In addition, an insulating layer 23b (e.g., polyimide, polyamide, polyvinylalcohol, polyparaxylylene, SiO, or SiO.sub.2) and a light-shielding film 19 consisting of aluminum or chromium are formed on the substrate 30a. A counter electrode 31 (ITO: Indium Tin Oxide) and an insulating film 32 are formed on a substrate 30b as a counter substrate.
A ferroelectric liquid crystal 33 is sandwiched between the substrates 30a and 30b. A sealing member 35 for sealing the ferroelectric liquid crystal 33 is formed around the substrates 30a and 30b.
Polarizers 29a and 29b in a state of crossed Nicols are arranged at two sides of the liquid crystal element having the above cell structure, and a reflecting plate 28 (a diffusion-reflecting aluminum sheet or plate) is located behind the polarizer 29b so that an observer A can observe a display state by reflected light I.sub.1 of incident light I.sub.0.
In FIG. 5, source and drain electrodes respectively corresponding to the terminals 18 and 21 of the TFT are named assuming that a current flows from the drain to the source. In an operation as an FET, the source can serve as the drain.